The invention relates to an electroluminescent device comprising a first electrode, a second electrode and an ionic, organic layer which is in contact with said first electrode, which layer contains a conjugated compound and mobile ions. The invention also relates to a method of manufacturing an electroluminescent device comprising an ionic layer, which layer contains mobile ions.
An electroluminescent (EL) device is a device built up of an electroluminescent layer, which layer emits light when a voltage is applied across electrodes which are in contact with said layer. Such a device can be used, inter alia, as a light source whose light output can be varied in a simple manner by varying the applied voltage. An assembly of independently addressable EL devices, for example in the form of a matrix of light-emitting areas, can be used as a display.
Apart from EL devices based on inorganic materials, such as GaAs, also EL devices based on organic materials are known. Organic EL (oEL) devices on the basis of low-molecular weight materials and on the basis of polymers are known. Known oEL devices are single-layer devices, which means that, apart from the electrodes, the device only comprises the electroluminescent layer, or they are multilayer devices.
The performance of an organic EL device, measured, for example, in terms of the luminance at a specific voltage, depends to a substantial degree on which electrode materials are used. In general, it is assumed that in the case of electrons, the number of electrons injected depends exponentially on the difference between the work function of the electrode and the electron affinity of the organic layer. In the case of holes, the difference between the work function of the electrode and the ionization potential of the organic layer is of corresponding importance. This dependence applies mutatis mutandis also to the EL efficiency, which is defined as the ratio between the number of photons emitted and the number of charge carriers injected, as said EL efficiency is governed by the ratio between the electron current and the hole current. Consequently, it has been found in practice that in the case of, in particular, single-layer devices, the performance necessary for the above-mentioned applications generally can only be achieved if the negative electrode, also referred to as cathode, comprises a metal having a low work function. A low work function is to be understood to mean herein a work function of approximately 3.0 eV or less. A known electrode material, i.e. calcium, meets this criterion. A disadvantage of such metals is that they degrade under the influence of air. Consequently, the service life of EL devices based on such metals is very limited under atmospheric conditions. A known measure to enable metals having a higher work function to be used as the cathode material consists in incorporating additional layers into the device. In general, the manufacture of such multilayer devices is laborious and expensive. Besides, the performance of the device still depends, in principle, on which electrode material is selected: the work function still has to be attuned to the ionization potential and electron affinity of the layers used. The layers and electrodes can only be optimized in conjunction with each other, not separately. Given the multitude of factors which determine the functionality of a layer, such as the layer thickness, the electrical conductivity, the ionization potential, the electron affinity, the band gap and the photophysics, the optimization of a multilayer device is laborious. Consequently, there is a clear need for a simple, single-layer oEL device, which permits electrodes having a high work function to be used without the performance of the device being adversely affected.
Such a device was described recently by Pei et. al., in Science (1995) vol. 269, 1086. In this known device, referred to as "light-emitting electrochemical cell" (LEC) by Pei et. al., an electrolyte, for example lithiumtrifluoromethanesulphonate, is added to a layer of a known electroluminescent material, such as a poly(phenylenevinylene), which causes, according to said publication, a p-n junction to be formed in situ by means of electrochemical doping of the EL material. This measure results, inter alia, in that the device emits light already at a voltage which corresponds approximately to the band gap of the EL material and in that EL efficiencies comparable to known polymer-based EL devices (pEL) are achieved while using electrode materials having a high work function such as gold and aluminium.
However, the known LEC has disadvantages. Although the known LEC makes use of electrodes having a high work function, this has no effect on the service life. Said service life is comparable to that of corresponding devices in which a cathode having a low work function is used instead of an electrolyte. As explained hereinabove, the service life of the latter devices under ambient conditions is very limited and definitely insufficient for the intended applications. A further disadvantages is that by means of diffusion the electrolyte can move through every organic layer while preserving its charge neutrality. Consequently, a multilayer construction of the known LEC in which only one layer contains the electrolyte is not feasible. In addition, it is difficult to disperse the electrolyte on a molecular scale in the customary EL materials, which, in general, are non-ionic and predominantly apolar.